1. Field of the Invention
The present invention relates to a method for manufacturing a SIMOX (Separation by IMplanted OXygen) wafer based on a SIMOX technology among methods for manufacturing an SOI (Silicon On Insulator) wafer having a single-crystal silicon layer formed on a silicon single-crystal main body via an oxide film.
2. Description of the Related Art
As conventional methods for manufacturing an SOI wafer, there are known a bonding method for bonding two silicon wafers via an oxide film and a SIMOX method for implanting oxygen (O+) ions from a silicon wafer surface to form an ion-implanted layer serving as an oxygen layer having a high concentration in a region at a predetermined depth in a wafer and applying a heat treatment to this wafer to change the ion-implanted layer into a buried oxide film (which will be referred to as a BOX layer hereinafter). In particular, an SOI wafer manufactured by the SIMOX method is called a SIMOX wafer.
A method for manufacturing a SIMOX wafer at an initial stage of development is based on a high-dose technology. In this method for manufacturing a high-dose SIMOX wafer, oxygen ions are implanted into a silicon wafer at the rate of approximately 2×1018 atoms/cm2 with an implantation energy of approximately 200 keV, a BOX layer is formed in the wafer in an ion-implanted state (as-implanted state), and then high-temperature annealing treatment is carried out. As a result, a crystal defect that has occurred in the SOI layer by this annealing treatment can be remedied, the BOX layer can be reformed, and an interface between the SOI layer and the BOX layer can be flattened.
However, since in this high-dose technology for manufacturing the SIMOX wafer an oxygen ion dose amount is large, there has been a problem that many threading dislocations occur in the SOI layer, an ion implantation time is long and a manufacturing efficiency is poor, for example. This threading dislocation brings a leak current or deterioration in a heterointerface when a device is manufactured. As a result, an improvement in device performance or development of functionality may be obstructed.
Therefore, a low-dose technology has been developed in order to inhibit threading dislocations from being generated in an SOI layer and to reduce an ion implantation time. In this method for manufacturing a low-dose SIMOX wafer, oxygen ions are implanted from a surface of a silicon wafer at the rate of approximately 4×1017 atoms/cm2 with an implantation energy of approximately 180 keV, and then a high-temperature heat treatment is carried out to form a continuous BOX layer. In a case where the implantation energy is 180 keV, the BOX layer that is continuous in parallel to the silicon surface can be formed only when a dose amount is approximately 4×1017 atoms/cm2. This dose amount is called a dose window. In this low-dose technology for manufacturing the SIMOX wafer, a density of threading dislocations in the SOI layer can be reduced and an implantation time can be reduced to improve a manufacturing efficiency.
However, since in this low-dose technology for manufacturing the SIMOX wafer an oxygen ion dose amount is small, a film thickness of the BOX layer becomes thin resulting in a problem of reliability of the BOX layer. Further, in a case where a film thickness of the BOX layer to be formed is small, when a particle adheres to a silicon wafer surface at the time of ion implantation, this particle functions as a mask so that an unimplantable part is apt to be generated in the ion-implanted layer formed in the silicon wafer. Since the ion-implanted layer becomes the BOX layer by annealing treatment, the ion unimplantable part serves as a pin hole that is one type of crystal defects of the BOX layer to reduce electrical insulation properties. There has been a problem that a percentage of this pin hole density is higher than that of a high-dose SIMOX wafer, for example.
Thus, in order to inhibit generation of the pin hole in the BOX layer, there have been proposed a method for manufacturing an SOI substrate called an ITOX (Internal Thermal OXidation) technology where annealing treatment is performed to an ion-implanted silicon wafer and then oxidizing treatment is carried out in a high-temperature oxygen atmosphere, and an SOI substrate manufactured thereby (see, e.g., Japanese Unexamined Patent Application Publication No. 263538-1995 (claim 1, claim 3, claim 6, paragraphs [0009], [0010], [0025], and [0026], FIG. 1)).
In the method disclosed in Unexamined Patent Application Publication No. 263538-1995, an ion-implanted layer formed in a wafer is subjected to annealing treatment in an inert gas atmosphere containing oxygen having a concentration less than 1.0% to be changed into a BOX layer, and then this wafer is further subjected to high-temperature treatment in an atmosphere containing oxygen having a high concentration exceeding 1%. High-concentration oxygen in the atmosphere is diffused to the inside from front and back sides of the wafer by the high-temperature heat treatment, and stays to be laminated as SiO2 at a BOX layer interface portion. As a result, the BOX layer grows, and a film thickness of the BOX layer can be increased. In this ITOX technology manufacturing a SIMOX wafer, since SiO2 is laminated in the BOX layer, a pin hole generated in the BOX layer can be remedied, thus reducing a pin hole density. Further, root mean square roughness (Rms) of an interface between the BOX layer and the silicon wafer can be improved. As a result, electrical characteristics of a device can be homogenized.
However, even in this ITOX technology manufacturing SIMOX wafer, since an oxygen ion dose amount is large, an ion implantation time is long. Moreover, a high-temperature oxidizing treatment is required in addition to the annealing treatment, and hence there is a problem that the manufacturing efficiency is poor and the productivity is lowered.
Thus, in order to reduce the ion implantation time, there has been proposed a method for manufacturing an SOI wafer called an MLD (Modified Low Dose) technology where oxidizing treatment is applied to a wafer having two ion-implanted layers including an amorphous layer formed therein (see, e.g., a specification in U.S. Pat. No. 5,930,643 (claim 1, specification p. 1, a second column on the right side, ll. 5 to 43, FIG. 1(a)
In the method disclosed in U.S. Pat. No. 5,930,643, an ion is implanted while changing a temperature of a wafer to form two ion-implanted layers in different states, i.e., a high-concentration oxygen layer and an amorphous layer in the wafer, and a high-temperature oxidizing treatment is applied to the wafer in a mixed gas atmosphere.
Specifically, in a state where the silicon wafer is heated, oxygen ions are implanted at the rate of 2×1017 atoms/cm2 with an implantation energy of 185 keV to form a first ion-implanted layer as the high-concentration oxygen layer in the wafer. Then, in a state where this wafer is cooled, the oxygen ions are implanted at the rate of 3×1014 atoms/cm2 with the implantation energy of 185 keV to form a continuous second ion-implanted layer in an amorphous state on the first ion-implanted layer. Further, a temperature of this wafer is increased in an inert atmosphere, e.g., argon containing oxygen, and then high-temperature oxidizing treatment of maintaining the wafer at a high temperature in an oxidizing atmosphere containing oxygen and argon in the ratio of 40% to 60% is carried out. Maintaining the wafer at a high temperature in this oxidizing atmosphere changes the first ion-implanted layer into a BOX layer. Furthermore, the second ion-implanted layer partially overlaps the first ion-implanted layer, and hence the amorphous layer contains high-concentration oxygen. Therefore, re-crystallization of the second ion-implanted layer does not smoothly advance at the time of increasing a temperature in the inert atmosphere, and the second ion-implanted layer becomes a high-density defective layer including polycrystal, a twin crystal, or a stacking fault. Oxygen readily precipitates in a region in which this defective layer is formed. In a subsequent process of maintaining the wafer at a high temperature in the oxidizing atmosphere, oxygen in the atmosphere enters the wafer from front and back sides of the wafer to be dispersed inside, and oxygen is concentrated in the high-density defective layer topresipitate, thereby forming the BOX layer. Therefore, a small ion dose amount allows acquisition of a SIMOX wafer having a BOX layer whose thickness is the same as that obtained when implanting a double dose amount.
However, in the manufacturing method disclosed in U.S. Pat. No. 5,930,643, oxygen in the oxidizing atmosphere during the high-temperature oxidizing treatment is apt to be precipitated in the high-density defective layer, and a SIMOX wafer having a BOX layer with a thin film thickness demanded by a semiconductor device manufacturer is hard to be manufactured.
As a countermeasure, a temperature in the high-temperature oxidizing treatment is further increased to resolve oxygen precipitated in the high-density defective layer, or an oxygen concentration in the oxidizing atmosphere is lowered to reduce an amount of oxygen concentrated in the high-density defective layer, whereby an oxygen precipitation amount is reduced, thus forming a BOX layer with a thin film thickness. However, considering performance of a current anneal furnace, when a high-temperature treatment is carried out at a temperature exceeding 1350° C., a high-quality SIMOX wafer is hard to be manufactured. Moreover, when an oxygen concentration in the oxidizing atmosphere is lowered, an oxidizing treatment time becomes longer in order to obtain an SOI layer having a predetermined film thickness, resulting in a problem that a manufacturing efficiency is deteriorated and productivity is decreased.